This invention relates to a circuit and method for the acquisition of a voltage signal representing positional information and the conversion thereof into an electrical signal compatible with the analog input channels of a computer interface, so that the voltage signal may be used for controlling character selection on a cathode ray tube (CRT) screen, point identification on diagrams projected onto a CRT screen, or the movement of cursors on a CRT screen, and similar applications. More particularly, this invention relates to the electrical circuitry for interfacing a touch tablet of the type described in co-pending United States Patent application Ser. No. 475,418 filed Mar. 15, 1983, now U.S. pat. No. 4,484,026, with visual displays such as video game displays or computer monitor screens.
Numerous devices, such as personal computers, video games, and monitors containing a cathode ray tube (CRT), are equipped with analog input channels. A device of this type is referred to herein as a "computer interface".
Position sensors for use in controlling the x-y coordinates of a point on a CRT screen are well known. Such systems are described, for example, in D. Thornburg, U.S. Pat. No. 4,313,113; Kley, U.S. Pat. No. 4,079,194; Kley, U.S. Pat. No. 4,214,122; Hurst, et al., U.S. Pat. No. 3,911,215; Hurst, U.S. Pat. No. 3,798,370; and Yokoo, et al., U.S. Pat. No. 4,319,078, which are incorporated herein by reference. Position sensors may be classified into two basic categories depending on the nature of the output of the position sensor.
One type of position sensor provides an electrical resistance whose value represents position information. An example of this type of position sensor is described in Yokoo, et al., U.S. Pat. No. 4,319,078, wherein a position sensor provides two resistances whose magnitudes are proportional to the x and y coordinates, respectively, of a selected point within a two-dimensional medium in the position sensor. These external resistances are then coupled to circuitry in a computer interface which converts each resistance value into a digital number for further use by the computer interface.
A second type of position sensor, as illustrated by Hurst, U.S. Pat. No. 3,798,370, provides an output voltage signal which is derived from position information.
The invention disclosed herein is concerned with position snesors of the second type, i.e., those which provide voltage signals representative of position information. However, before explaining the invention disclosed herein, it will be useful to explain how a typical computer interface processes the positional information made available by a position sensor of the first type.
FIG. 1 shows a section of a typical computer interface 3. Computer interface section 3 is suitable for coupling an external resistance 10 provided by a position sensor of the first type between input lead 5 and input pin P.
A typical computer interface has one or more sections identical to the section shown in FIG. 1, except that a single bit rate clock 4 is common to all sections.
If a position sensor of the first type makes two resistances available, an x-resistance and a y-resistance, representing the x and y coordinates respectively of a point on a two dimensional medium, then the computer interface is assumed to have at least two sections, an x-section and a y-section identical to the computer interface section 3 shown in FIG. 1 for coupling to the x-resistance and the y-resistance respectively. In the two dimensional cases, the elements shown in FIG. 1 will be denoted by prefixing an x- or y- to the element named (except for bit rate clock 4, which is assumed to be a common clock).
In general if a position sensor of the first type makes N resistances available representing the coordinates of a point in N dimensional space, then the computer interface is assumed to have N sections identical to the section shown in FIG. 1, where each of the N resistances 10 is coupled between input lead 5 and input pin P of a corresponding section.
Computer interface section 3 contains a lead 5 which is connected at one end to a supply voltage having a magnitude V.sub.cc. Voltage divider 17 is connected between lead 5 and ground. The output signal of voltage divider 17 serves as a first input signal on input lead 18 of voltage comparator 13. Capacitor 11 is connected between lead 16 and ground. One end of lead 16 serves as a second input lead of comparator 13 and the other end of lead 16 is connected to an input pin P via internal resistor R. R is typically between 100 ohms and 2.2 K ohms.
When an external resistance 10 provided by a position sensor of the first type (shown as an input paddle in FIG. 1) is coupled between lead 5 and input pin P, capacitor 11 within computer interface section 3 is charged through the variable resistor 10 of the input paddle. Variable resistor 10 typically has a value between 60 K ohms and 1 megohm.
The precise operation of computer interface section 3 depends on whether the computer interface is synchronous or asynchronous. For a synchronous computer interface, such as those made by Atari and the Commodore 20, the charging and discharging cycle of internal capacitor 11 contains a charging phase of a fixed length of time followed by a clamping phase of fixed length of time. The charging and discharging cycle is short, typically 1/60 second. The internal capacitor 11 is charged during the charging phase and is discharged during the clamping phase.
The count in digital counter 15 is set to zero during the discharge phase. During the charging phase the count in counter 15 is incremented by one at a fixed bit count rate from 0 until a fixed positive integer denominated Max Count is reached or until the voltage on capacitor 13 reaches a selected threshold voltage, whichever occurs first. The output signal from bit rate clock 4 is provided to counter 15 via gate circuit 14. The bit count rate and the period of the charging phase are selected so that the bit count rate times the period of the charging phase equals Max Count.
When the voltage on input line 16 of comparator 13 reaches the threshold voltage, V.sub.th, (defined below) comparator 13 supplies a signal to gate circuit 14. The output signal of gate circuit 14 is then provided to counter 15 on lead 14b, causing the count in counter 15 to be provided on lead 15a to other circuitry (not shown) in the computer interface. As will be shown below, this count is indicative of the resistance value 10, which in turn represents a desired position on the display.
The companion input signal on lead 18 of comparator 13 is the output signal of voltage divider 17 which provides a voltage between V.sub.cc and ground. The setting of divider 17 establishes the magnitude of the threshold voltage, V.sub.th, which in turn establishes the level which the voltage on lead 16 from capacitor 11 must reach in order to trigger comparator 13.
When an external resistor 10 of suitable magnitude provided by a position sensor of the first type is inserted between lead 5 input pin P, the voltage, V, across capacitor 11 rises according to the formula V=V.sub.cc (1-e.sup.-t/RC) as illustrated in FIG. 9 (where the dotted line indicates the threshold voltage V.sub.th). The time required for the voltage across capacitor 11 to attain the threshold voltage, V.sub.th depends upon the values for R, C and k according to the well known equation EQU t=RCk
where
R=value of external resistor 10; PA1 C=capacitance of capacitor 11; PA1 k=a constant depending on the threshold and supply voltages, V.sub.th and V.sub.cc, respectively, and equals ln (V.sub.cc /(V.sub.cc -V.sub.th)).
Thus the count in counter 15 when the voltage across capacitor 11 reaches V.sub.th is a digital representation of the time required for the voltage across capacitor 11 to rise from 0 to V.sub.th. In turn the time required is directly proportional to the resistance R, which represents positional information.
An asynchronous computer interface operates similarly to the above described synchronous computer interface except that it typically operates on demand and except that as soon as comparator 13 signals gate circuit 14 that the voltage across capacitor 11 has reached V.sub.th, an output signal from gate circuit 14 on lead 14a turns on clamping transistor 12 which discharges capacitor 11. The charging rate is sufficiently high that capacitor 11 is charged to the threshold voltage V.sub.th many times a second. The output signal of gate 14 is simultaneously provided to counter 15 on lead 14b so that the count on counter 15 is provided on lead 15a to other circuitry (not shown) within a computer or other system.
The values of the constants C and V.sub.th used in the equation t=RCk are specified by the manufacturer. For example, in a typical Apple computer, V.sub.th =0.66 V.sub.cc. Of importance, however, the time it takes the voltage across the internal capacitor in the computer interface to reach V.sub.th depends on C and V.sub.th as actually found in a particular computer interface, and not upon the nominal values specified by the manufacturer. Hence, the prior art method of deriving a measure of position information from the time required for the voltage across the internal capacitor to rise above V.sub.th is sensitive to variations in the actual values of C and V.sub.th in a specific system from nominal values.
In contrast, the interface circuitry of my invention provides a measure of position information which is virtually independent of these variations.